The intrinsically X-ray-weak quasar PHL 1811. II. Optical and UV spectra and analysis

This is the second of two papers reporting observations and analysis of the unusually bright (m(b) 14: 4), luminous (M-B = -25.5), nearby (z = 0.192) narrow-line quasar PHL 1811. The first paper reported that PHL 1811 is intrinsically X-ray-weak and presented a spectral energy distribution ( SED). Here we present HST STIS optical and UV spectra, and ground-based optical spectra. The optical and UV line emission is very unusual. There is no evidence for forbidden or semiforbidden lines. The near-UV spectrum is dominated by very strong Fe II and Fe III, and unusual low-ionization lines such as Na I D and Ca II H and K are observed. High- ionization lines are very weak; C IV has an equivalent width of 6.6 8, a factor of similar to 5 smaller than measured from quasar composite spectra. An unusual feature near 1200 8 can be deblended in terms of Ly alpha, N V, Si II, and C III* using the blueshifted C IV profile as a template. Photoionization modeling shows that the unusual line emission can be explained qualitatively by the unusually soft SED. Principally, a low gas temperature results in inefficient emission of collisionally excited lines, including the semiforbidden lines generally used as density diagnostics. The emission resembles that of high-density gas; in both cases this is a consequence of inefficient cooling. PHL 1811 is very unusual, but we note that quasar surveys may be biased against finding similar objects.